A method of the above type has been disclosed in IBM Technical Disclosure Bulletin Vol 32 No. 5B, pages 355-356. In the known method use is made of at least one intermediate substrate which comprises a hard core layer provided with an adhesive layer at least at the side facing the conductive traces of the base substrate. The method serves to substantially eliminate the dimensional instability that usually occurs in composite lamination processes. While this can be recognized as a substantial improvement in the manufacture of multilayer boards, the disclosure fails to address an even more important problem associated with multilayer boards, namely that of providing a material displaying thermal coefficients of expansion (TCE) sufficiently low so as to match the TCE of electronic components (chips) used in conjunction with the multilayer board. A woven glass fabric (cloth) being used as the reinforcement material it is immediately apparent to the person of ordinary skill in the art that the TCEs obtained are relatively high. Further, the prior art substrates and the resulting multilayer boards require improved dimension stability.
Similar considerations apply to U.S. Pat. No. 3,756,891, which discloses a method of manufacturing multilayer PWBs involving the stacking of circuitized boards with adhesive coated sheets. The adhesive is chosen so as not to flow into the through-hole interconnection areas present in the boards.
A different approach towards multilayer PWBs is the sequential laminating technique disclosed in RCA review 29 (1968) pages 582-599, particularly pages 596-597. Although a base-substrate provided with circuitry on both sides is laminated with an adhesive coated dielectric layer, the adhesive coated layer is not an intermediate substrate in between base substrates in accordance with the invention, but serves as a substrate for a next printed circuit. The disclosure does not address the type of substrate used, let alone that it can provide a solution to the problem of providing multilayer boards having sufficiently low TCEs.
PWBs providing advantages with respect to TCE have been disclosed in U.S. Pat. No. 4,943,334. Described is a manufacturing process which comprises winding reinforcing filaments about a square flat mandrel to form a plurality of layers of filaments intersecting at an angle of 90.degree., providing the plurality of layers with a curable matrix material, and curing the matrix so as to form a base material for a PWB. In order to provide multilayer PWBs the disclosure teaches a method comprising providing an assembly of PWBs in a cavity, introducing a curable matrix material into the cavity, and curing the matrix so as to form a multilayer PWB. The desired reinforcement of the matrix is obtained by the presence of fibres around the PWBs, which during the process will become embedded in the cured matrix. The method fails to provide acceptable suitable results due to, inter alia an internal lack of thickness-tolerance.
In C. J. Coombs, Printed Circuits Handbook, published by McGraw-Hill, chapters 31 and 32, more particularly 33 and 34, it is described, int. al., how a multiple layer printed wire board, a so-called multilayer, is generally manufactured, the process being comprised of the following steps:
manufacturing a laminate coated on both sides with copper foil from glass fabric-epoxy prepreg; PA1 etching the desired pattern into the copper; PA1 bonding the etched laminates by pressing them together with intermediate layers of glass fibre-epoxy prepreg.
There are a number of drawbacks to this process, such as high materials costs on account of glass fabric being employed and high thermal expansion on account of the low maximum fibre content in fibre-reinforced laminates. Another major drawback to this process is that there is no absolute thickness tolerance. The thickness of a multilayer formed in this manner is dependent on, int. al., the moulding pressure exerted, the moulding temperature and the warming-up rate employed, and the "age" of the used prepreg and some other factors which are hard to control.
There are several variations from the latter process, e.g., as disclosed in EP 0 231 737 A2. In this known process a multilayer printed wire board is manufactured in a continuous process. In the embodiment according to FIG. 2 of this publication use is made of a single printed wire board (PWB) comprised of a substrate of two layers of glass cloth in a cured matrix of thermosetting synthetic material, which substrate is provided on both sides with a layer of copper traces formed by the subtractive method from the copper foil originally applied to the substrate. To this initial PWB there are applied, on both sides, two layers of glass cloth, a layer of liquid thermosetting material, such as epoxy resin, and a copper foil. After preheating the whole is laminated in a double belt press under the effect of heat and pressure. Thus, after cooling as it leaves the double belt press, a laminate is obtained which after the forming of copper traces in the outer layers makes a multilayer PWB. Hence this multilayer PWB is made up of a laminate of three substrates of glass cloth-reinforced cured epoxy resin and four layers with copper traces.
Although quite reasonable results can be obtained using the multilayer PWB manufactured according to this known process, it still has certain drawbacks. Notably, the layers of liquid, not yet cured thermosetting resin are greatly pressed together in the double belt press, as a result of which there is a substantial decrease of the laminate's thickness between the double belt press's inlet and its outlet. It has been found that as a result of this major change in thickness it is hard to maintain with sufficient accuracy the constant thickness of the finished laminate and of the finished multilayer PWB as ultimately desired. Deviations in a PWB's thickness have an unfavourable effect on its electrical properties, thus negatively affecting the quality of such a PWB. Another drawback to said known multilayer PWB is that reinforcing the substrates with fabrics is a comparatively costly affair.
DE-4 007 558 A1 describes a multilayer PWB of a somewhat different type. Between a number of adjacent single PWBs (cf. FIG. 1, no. 2 of DE-4 007 558 A1) which are each composed of a substrate (cf. FIG. 1, no. 4) made up of a glass cloth impregnated with a thermosetting synthetic material and provided on both sides with copper traces (of FIG. 1, no. 5), there is interposed in each case a sort of intermediate substrate (FIG. 1, nos. 1-a and 1-b). The intermediate substrate (1) consists in this case of a polyimide film (1-a) of a thickness of 10 .mu.m which is provided on both sides with an adhesive layer (1-b) of a thickness of 10 .mu.m or less. The melting temperature of the polyimide film is higher than the temperature used during lamination, while the adhesive layers have a melting temperature below the used lamination temperature.
A disadvantage of said known multilayer PWB consists in that there is air in the voids between the copper traces (of FIG. 1), which may have an unfavourable effect on the properties. Other disadvantages of DE-4 007 558 A1 include the high materials cost price of the described constituents and the lengthy processing time required.
In U.S. Pat. No. 4,606,787 a process for manufacturing a multilayer PWB is described which comprises first (cf. FIG. 12) making a stack of a number of single PWBs with sandwiched therebetween in each case a sort of intermediate substrate of glass fibres impregnated with liquid, uncured epoxy resin. Next, said stack is pressed together under pressure and at elevated temperature, with the resin filling the voids between the conductive traces (cf. column 6, ll. 51, 52) and being cured. The pressing together of the laminate gives a substantial reduction of its thickness, making it difficult to maintain with sufficient accuracy the constant overall thickness of the finished laminate as ultimately desired and the constant thickness of the individual intermediate substrates. This has an unfavourable effect on the PWB's electrical properties, thus negatively affecting its quality.
A multilayer printed wire board in which the dielectric material is formed of UD-reinforced layers is also disclosed in JP-A-1,283,996. The disclosed multilayers are based on the lamination of unidirectionally oriented parallel fibres (UD) containing prepreg, and so suffer from the problem that the orientation may deteriorate. Retaining orientation is crucial to enjoying the proper benefits of UD reinforcement.
Copending, not prepublished International patent application PCT/EP92/01133 (publication number WO 92/22192) (U.S. application Ser. No. 08/157,077, now U.S. Pat. No. 5,592,737) incorporated by reference herein for all purposes, provides a method in which said drawbacks have been obviated. The method described consists in that, use being made of a hard base substrate having conductive traces on both sides and an intermediate substrate comprising a hard core layer coated with an adhesive layer that is flowable at least at the side facing the conductive traces of the base substrate, lamination is conducted under a pressure sufficiently high so as to bring the core layer of the intermediate substrate into contact or virtually into contact with the conductive traces of the base substrate, the adhesive filling the voids between the traces, the base substrate and the intermediate substrate comprising a fibre-reinforced matrix material, the reinforcement being in the form of a crosswise arrangement of layers of unidirectionally (UD) oriented fibres.